Cables are omnipresent in all electrical systems, for the supply of power or transmission of information. These cables are subject to the same stresses as the systems that they connect and may be subject to failures. It is therefore necessary to be able to analyze their state and to return information on the detection of faults, but also on their location and their type, in order to help with maintenance. Conventional reflectometry methods allow this type of test to be carried out.
Reflectometry methods employ a principle that is close to that of radar: an electrical signal, the probe signal, often of high-frequency or wideband, is injected at one or more points into the cable to be tested. Said signal propagates through the cable or network and some of its energy is reflected when it encounters an electrical discontinuity. An electrical discontinuity may for example result from a connection, from the end of the cable or from a fault, or more generally from an abrupt change in the propagation conditions of the signal in the cable. Discontinuities most often result from faults that modify the characteristic impedance of the cable locally, thereby creating a discontinuity in its linear parameters.
The analysis of the reflected signals at the point of injection makes it possible to deduce therefrom information on the presence and location of these discontinuities, and therefore of any faults. An analysis in the time or frequency domain is conventionally carried out. These methods are designated by the acronyms TDR (for time-domain reflectometry) and FDR (for frequency-domain reflectometry).
The invention applies to any type of electric cable, in particular power transmission cables or communication cables, whether in fixed or mobile installations. The cables in question may be coaxial cables, twin-lead cables, parallel-line cables, twisted-pair cables or any other type of cable provided that it is possible to inject a reflectometry signal into it and to measure its reflection.
Known time-domain reflectometry methods are particularly suitable for detecting, in a cable, hard faults such as a short-circuit or an open circuit or more generally any significant local modification of the impedance of the cable. A fault is detected by measuring the amplitude of the signal reflected from this fault, which increases and therefore becomes more detectable as the hardness of the fault increases.
In contrast, a soft defect, for example resulting from a superficial degradation of the cladding, insulator or conductor of the cable, generates a low-amplitude peak in the reflected reflectometry signal and therefore one that is not easily detectable with conventional time-domain methods.
The detection and location of soft defects in cables is a substantial problem for the industrial world because a fault in general first appears as a superficial fault but may, with time, progress to become more severe. For this reason in particular, it is useful to be able to detect the appearance of a fault as soon as it appears and at a stage at which its impact is superficial, in order to prevent it from becoming more severe.
Known methods allowing soft defects to be identified in a cable are most often time-frequency domain reflectometry methods. These methods were developed in order to allow better detection of low-amplitude reflected signals.
In particular, the “Joint Time-Frequency Domain Reflectometry” method described in document [1], which proposes the use of a Wigner-Ville frequency transform, is known. This method allows better discrimination between signals reflected from soft defects, with a good time and frequency resolution. However it has the two-fold drawback of being complex to implement in an on-board system and of causing problems with false detections due to the existence of cross terms in the aforementioned transform.
The French patent application of the Applicant published under the number FR 2981752 proposes an improvement to the time-frequency method described in [1], allowing the influence of the cross terms to be eliminated and the problems with false detections to be solved.
However, this method still has the drawback of substantial complexity with regard to implementation in portable pieces of equipment.
Moreover, the French patent application of the Applicant published under number FR 2907910 is also known, this document providing another analyzing method based on injection of pseudo-random sequences into the cable to be tested.
The invention proposes a method for analyzing a cable with a view to detecting soft defects, which has a low implementational complexity and which is particularly suitable for implementation in a portable and/or on-board device.
The method according to the invention allows very low-amplitude signals to be detected even when the measurement environment is noisy and other reflections of higher amplitude are present.
The objective targeted by the invention is achieved by virtue of the use of an intercorrelation function normalized by a particular coefficient allowing the obtained reflectogram to self-adapt in order to make low-amplitude reflections stand out.